Back-up uninterruptible power system

Abstract

A back-up uninterruptible power system has a power supply path from input terminals connected to AC power system lines to normally supply power to a load. Upon the occurrence of a line fault, a static switch in the power supply path interrupts the connection between the AC power lines and the load and an inverter is turned on to provide power derived from an auxiliary battery through a transformer to the power supply path to supply AC power to the load. By using the static switch, switching from line connection to backup power can be done quickly, within a half cycle, so that substantially no interruption of the output waveform is observed. The inverter can be operated to provide a commutation pulse to the SCRs in the static switch to commutate an SCR which might otherwise continue conducting after the triggering signals to the gates are cut off and before the inverter supplies the AC power to the load. A relay connected between the input terminals and the static switch is controlled to open relay contacts after the static switch has interrupted the current flow to the load, thereby providing positive electrical isolation of the AC power system from the uninterruptible power system while allowing the mechanical relay contacts to switch only during times of no current. Line fault detection is carried out by creating a digital waveform composed of an average of prior cycles of the AC input waveform, converting the stored waveform to an analog signal, and comparing it with the present input signal in phase lock therewith to detect deviation of the input signal from the reference waveform.

What is claimed is: 
     
       1. AC line fault detection apparatus comprising: (a) means for sampling an AC power line input voltage at periodic times and providing digital output data corresponding to the sampled input voltage;   (b) means receiving the digital data corresponding to the sampled AC power line voltage and for producing a reference waveform at selected sample times during the period of the AC line voltage, wherein each reference value at each sample time comprises a selectively weighted average of sample values of the cycle of the AC waveform presently being sampled and prior cycles of the AC power voltage signal;   (c) means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power voltage signal;   (d) digital to analog conversion means receiving the digital reference waveform data for providing an analog reference waveform signal corresponding thereto;   (e) means for providing a selected relative scaling of the magnitudes of the AC power line voltage and the analog reference waveform signal to provide a selected magnitude tolerance band between the reference signal and the AC power line voltage;   (f) a comparator receiving the scaled AC power line voltage and the analog reference waveform signal at its inputs and providing an output signal indicating a line fault if the difference in magnitude between the two input signals is greater than the selected tolerance band.   
     
     
       2. The apparatus of claim 1 including means for providing a guard pulse signal to one of the inputs to the comparator for a selected period of time around the anticipated zero crossings of the reference voltage and the AC input voltage to prevent the comparator from changing its output state at times adjacent to such zero crossings. 
     
     
       3. The apparatus of claim 1 wherein the AC power line input voltage is rectified to provide only unidirectional voltage to the comparator and the reference voltage waveform is provided to the comparator as a unidirectional voltage. 
     
     
       4. The apparatus of claim 1 wherein the means for providing a selected scaling provides a biasing voltage to one input of the comparator to provide a selected voltage tolerance band between the AC input signal and the reference signal. 
     
     
       5. The apparatus of claim 4 wherein the comparator further includes a feedback path from the output of the comparator to one of the inputs of the comparator through a resistance to increase the allowable tolerance band between the AC input signal and the reference signal during normal operation where no line fault has occurred, and to decrease the tolerance band during occurrence of a line fault wherein the AC line voltage compared to the reference voltage was outside of the tolerance band, whereby the line fault output signal from the comparator will be cleared only if the AC input signal returns to a level which is within the narrower tolerance band about the reference waveform signal. 
     
     
       6. An uninterruptible power system connectible to AC power system lines at its input terminals and to a load at its output terminals, comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from an AC power system connected to the input terminals to a load connected to the output terminals;   (c) means responsive to control signals for interrupting the supply of power through the power supply path from the AC power system lines to the load and for selectively providing AC power from the battery to the power supply path and thence to the load;   (d) control means for providing control signals, upon occurrence of a line fault on the AC power system lines, to the means responsive to control signals to cause the supply of power to the load to be transferred from the AC power system to the battery, the control means including: (i) means for sampling the voltage on the AC power lines at the input terminals at periodic times and providing digital output data corresponding to the sampled input signal;   (ii) means receiving the digital data corresponding to the sampled AC power line signal and for producing a reference waveform at selected sample times during the period of the AC line signal, wherein each reference value at each sample time comprises a selectively weighted average of sample values of the cycle of the AC waveform presently being sampled and prior cycles of the AC power line voltage;   (iii) means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power line voltage;   (iv) a digital to analog conversion means receiving the digital reference waveform data for providing an analog reference waveform signal corresponding thereto;   (v) means for providing a selected relative scaling of the magnitudes of the AC power line voltage and the analog reference waveform signal to provide a selected magnitude tolerance band between the reference signal and the AC power line signal;   (vi) a comparator receiving the scaled AC input signal and the reference waveform signal and providing an output signal indicating a line fault if the difference in its inputs is greater than the selected tolerance input band.     
     
     
       7. The uninterruptible power system of claim 6, including means for providing a guard pulse signal to one of the inputs of the comparator for a selected period of time around the anticipated zero crossings of the reference voltage and the AC line voltage to prevent the comparator from changing its input state at times adjacent to such zero crossings. 
     
     
       8. The uninterruptible power system of claim 6, wherein the AC power line voltage is rectified to provide only unidirectional voltage to the comparator and the reference voltage waveform is provided to the comparator as a unidirectional voltage. 
     
     
       9. The uninterruptible power system of claim 6, wherein the means for providing a selected scaling provides a biasing voltage to one input of the comparator to provide a selected voltage tolerance band between the AC power line voltage and the reference signal. 
     
     
       10. The uninterruptible power system of claim 9, wherein the comparator further includes a feedback path from the output of the comparator to one of the inputs of the comparator through a resistance to increase the allowable tolerance band between the AC power line voltage and the reference signal during normal operation where no line fault has occurred, and to decrease the tolerance band during occurrence of a line fault wherein the AC line voltage compared to the reference voltage was outside of the tolerance band whereby the line fault output signal from the comparator will be cleared only if the AC line voltage returns to a level which is within the narrower tolerance band about the reference waveform signal. 
     
     
       11. The uninterruptible power system of claim 6, wherein the means responsive to control signals includes an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the battery and to provide AC power across the other side of the bridge to the power supply path and including filter means in the power supply path between the inverter and the output terminal for low pass filtering and the output voltage from the inverter and wherein the control means controls the switching devices of the inverter in a pulse-width modulated manner to provide a substantially sinusoidal output voltage waveform at the output terminals with the width of the pulses provided from the inverter selected to provide a desired output waveform, the waveform selected to match the waveform of the AC line power at the time of line power fault detection so that a substantially continuous output voltage waveform is provided at the output terminals during switch-over from AC line power to power from the inverter. 
     
     
       12. The uninterruptible power system of claim 11 wherein the means responsive to control signals includes a transformer having its secondary connected in the power supply path and its primary connected to the inverter bridge to receive AC power from the inverter bridge. 
     
     
       13. The uninterruptible power system of claim 11 further including means for sensing the output current supplied from the output terminals to the load, and wherein the desired output waveform provided by the control means is the reference waveform compensated by the measured current being drawn by the load. 
     
     
       14. The uninterruptible power system of claim 13 wherein the desired waveform VREF* is determined in accordance with the following expressions:   VREF*=VREF+R(IO+)-R(IO-)     when the reference voltage corresponds to the positive halfcycle, and     VREF*=VREF-R(IO+)+R(IO-)     when the reference voltage corresponds to the negative half cycle and wherein IO+ is the sensed output current in a positive direction, IO- is the sensed output current in a negative direction, and R is a resistive scaling factor.   
     
     
       15. The uninterruptible power system of claim 11 wherein the inverter H-bridge has four arms with switching devices in upper and lower arms of the bridge and with the DC input to the bridge being provided across the upper and lower arms, and with the output of the bridge being taken between the junctions between the switching devices in the upper and lower arms on each side of the bridge, and wherein each of the gate controlled switching devices comprises a plurality of FETs with the number of FETs in the switching devices in each arm of the upper arms of the bridge being less than the number of FETs in the switching devices in each of the lower arms of the bridge, and wherein the control means controls the switching of the FETs such that between pulses of output power provided from the inverter to the power supply path, the control means turns off the FETs in the upper arms of the bridge and turns on FETs in the lower arms of the bridge so as to allow current flowing to the load to freewheel through the switching devices in the lower arms of the bridge, whereby the overall duty cycle for individual FETs in the upper and lower arms of the bridge over a full cycle of the inverter tends to be equalized. 
     
     
       16. The uninterruptible power system of claim 15 including means for comparing the voltage across the switching devices in the upper arms of the H-bridge inverter to a reference voltage indicative of a maximum current flow through the switching devices and therefore a maximum voltage drop across the switching devices, and providing an output signal if the voltage across the switching devices exceeds the reference, the control means terminating the pulse of the pulse width modulation by turning off the switching devices when the current limit signal occurs. 
     
     
       17. An uninterruptible power system connectible to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) means connected to receive DC power from the auxiliary battery, and for selectively providing AC power to the power supply path between the static switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path;   (e) control means, connected to provide control signals to the SCRs of the static switch means and to the gates of the gate controlled switching devices of the inverter, and including means for sensing the voltage at the input terminals to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the gate drivers of the SCRs to inhibit triggering the the SCRs and providing control signals to the gates of the switching devices of the inverter bridge to provide a pulse of voltage to the power supply path to reverse bias an SCR of the static switch that may be conducting at that time, and thereafter providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system; and   (f) a relay having mechanical relay contacts connected in the power supply path between the input terminals and the static switch means wherein the relay contacts are opened or closed in response to a control signal provided to the relay, and wherein the control means provides a control signal to the relay to open the relay contacts when a line fault is detected after the control means has controlled the static switch means to turn off the SCRs in the static switch means, and wherein the control means provides control signals to the relay to close the mechanical contacts of the relay before the control means provides control signals to the SCRs of the static switch means to turn them on to resupply power from the AC power system lines to the load after there is no longer a line fault on the AC power system.   
     
     
       18. The uninterruptible power system of claim 17 wherein the control means controls the switching devices of the inverter in a pulse-width modulated manner to provide voltage at the output terminals with the width of the pulses provided from the inverter selected to provide a desired output waveform selected to match the waveform of the AC input power at the time a line power fault is detected, so that a substantially continuous voltage output waveform is provided at the output terminals during switch-over from AC line power to power from the inverter, and wherein if the line fault is detected during a selected period of time around the zero crossing of the AC input waveform, the control means provides a control signal to the inverter to reverse the polarity of the normal power pulse that would be provided by the inverter to force commutate off the SCR in the static switch which is conducting. 
     
     
       19. An uninterruptible power system connectible to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) means connected to receive DC power from the auxiliary battery, and for selectively providing AC power to the power supply path between the static switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path;   (e) control means, connected to provide control signals to the SCRs of the static switch means and to the gates of the gate controlled switching devices of the inverter, and including means for sensing the voltage at the input terminals to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the gate drivers of the SCRs to inhibit triggering of the SCRs and providing control signals to the gates of the switching devices of the inverter bridge to provide a pulse of voltage to the power supply path to reverse bias an SCR of the static switch that may be conducting at that time, and thereafter providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system, wherein the control means further comprises: (i) means for sampling the AC power line signal at the input terminals at periodic times and providing digital output data corresponding to the sampled input signal;   (ii) means receiving the digital data corresponding to the sampled AC power line signal and for producing a reference waveform at selected sample times during the period of the AC line voltage wherein each reference value at each sample time comprises a selectively weighted average of the sample value of the cycle of the AC waveform presently being sampled and prior cycles of the AC power line voltage;   (iii) means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power line voltage;   (iv) digital to analog conversion means receiving the digital reference waveform data for providing an analog reference waveform signal corresponding thereto;   (v) means for providing a selected relative scaling to the magnitudes of the AC power line voltage and the analog reference waveform signal to provide a selected magnitude tolerance band between the reference signal and the AC power line voltage;   (vi) a comparator receiving the scaled AC ower line voltage and the reference waveform signal and providing an output signal indicating a line fault if the difference in magnitude between the two signals is greater than the selected tolerance band.     
     
     
       20. The uninterruptible power system of claim 19 including means for providing a guard pulse signal to one of the inputs to the comparator for a selected period of time around the anticipated zero crossings of the reference voltage and the AC power line voltage to prevent the comparator from changing its state at times adjacent to such zero crossings. 
     
     
       21. The uninterruptible power system of claim 19 wherein the AC power line voltage is rectified to provide only unidirectional voltage to the comparator and the reference voltage waveform is provided to the comparator as a unidirectional voltage. 
     
     
       22. The uninterruptible power system of claim 19 wherein the means for providing a selected scaling provides a biasing voltage to one input of the comparator to provide a selected voltage tolerance band between the AC power line voltage and the reference signal. 
     
     
       23. The uninterruptible power system of claim 22 wherein the comparator further includes a feedback path from the output of the comparator through a resistance to increase the allowable tolerance band between the AC power line voltage and the reference signal during normal operation where no line fault has occurred, and to decrease the tolerance band during occurrence of a line fault wherein the AC line voltage compared to the reference voltage was outside of the tolerance band whereby the line fault output signal from the comparator will be cleared only if the AC line voltage returns to a level which is within the narrower tolerance band about the reference waveform signal. 
     
     
       24. An uninterruptible power system connectible to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) means connected to receive DC power from the auxiliary battery, and for selectively providing AC power to the power supply path between the static switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path, wherein the means connected to receive power from the auxiliary battery includes a transformer having its secondary connected in the power supply path between the static switch means and the output terminals and its primary connected to the inverter bridge to receive AC power from the inverter bridge;   (e) control means, connected to provide control signals to the SCRs of the static switch means and to the gates of the gate controlled switching devices of the inverter, and including means for sensing the voltage at the input terminals to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the gate drivers of the SCRs to inhibit triggering of the SCRs and providing control signals to the gates of the switching devices of the inverter bridge to provide a pulse of voltage to the power supply path to reverse bias an SCR of the static switch that may be conducting at that time, and thereafter providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system.   
     
     
       25. An uninterruptible power system connectible to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) means connected to receive DC power from the auxiliary battery, and for selectively providing AC power to the power supply path between the static switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path;   (e) control means, connected to provide control signals to the SCRs of the static switch means and to the gates of the gate controlled switching devices of the inverter, and including means for sensing the voltage at the input terminals to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the gate drivers of the SCRs to inhibit triggering of the SCRs and providing control signals to the gates of the switching devices of the inverter bridge to provide a pulse of voltage to the power supply path to reverse bias an SCR of the static switch that may be conducting at that time, and thereafter providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system, wherein the control means controls the switching devices of the inverter in a pulse-width modulated manner to provide voltage at the output terminals with the width of the pulses provided from the inverter selected to provide a desired output waveform selected to match the waveform of the AC input power at the time a line power fault is detected, so that a substantially continuous voltage output waveform is provided at the output terminals during switch-over from AC line power to power from the inverter, wherein if the line fault is detected during a selected period of time around the zero crossing of the AC input waveform, the control means provides a control signal to the inverter to reverse the polarity of the normal power pulse that would be provided by the inverter to force commutate off the SCR in the static switch which is conducting, and   wherein the control means includes means for sampling the AC power line voltage at the input terminals at periodic times and providing digital data corresponding to the sampled input, means receiving the digital data corresponding to the sampled power line signal and for producing a reference waveform at selected sample times during the period of the AC line voltage wherein each reference value at each sample time comprises a selectively weighted average of sample values of the cycle of the AC waveform presently being sampled and prior cycles of the AC power line voltage, means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power line voltage, and a digital to analog conversion means receiving the digital reference waveform data for providing an analog reference waveform signal corresponding thereto, and wherein upon detection of a power line fault the switching devices of the inverter are controlled in a pulse-width-modulated manner to provide an AC output waveform from the inverter which tracks a desired waveform comprising the reference waveform signal, and thereafter, the means receiving the digital data calculates reference waveform values at each sample time as a selectively weighted average of a pure sinusoidal waveform stored in a memory and the prior reference waveform and recalculates the reference waveform in this manner for each cycle of the output waveform from the inverter.   
     
     
       26. The uninterruptible power system of claim 25 further including means for sensing the output current supplied from the output terminals to the load, and wherein the desired output waveform provided by the control means is the reference waveform compensated by the measured current being drawn by the load. 
     
     
       27. The uninterruptible power system of claim 26 wherein the desired waveform VREF* is determined in accordance with the following expressions:   VREF*=VREF+R(IO+)-R(IO-)     when the reference voltage is in the positive half-cycle, and     VREF*=VREF-R(IO+)+R(IO-)     when the reference voltage is in the negative half-cycle and wherein IO+ is the sensed output current in a positive direction, IO- is the sensed output current in a negative direction, and R is a resistive scaling factor.   
     
     
       28. An uninterruptible power system connectible to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) means connected to receive DC power from the auxiliary battery, and for selectively providing AC power to the power supply path between the static switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path;   (e) control means, connected to provide control signals to the SCRs of the static switch means and to the gates of the gate controlled switching devices of the inverter, and including means for sensing the voltage at the input terminals to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the gate drivers of the SCRs to inhibit triggering of the SCRs and providing control signals to the gates of the switching devices of the inverter bridge to provide a pulse of voltage to the power supply path to reverse bias an SCR of the static switch that may be conducting at that time, and thereafter providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system, wherein the control means controls the switching devices of the inverter in a pulse-width modulated manner to provide voltage at the output terminals with the width of the pulses provided from the inverter selected to provide a desired output waveform selected to match the waveform of the AC input power at the time a line power fault is detected, so that a substantially continuous voltage output waveform is provided at the output terminals during switch-over from AC line power to power from the inverter, and wherein if the line fault is detected during a selected period of time around the zero crossing of the AC input waveform, the control means provides a control signal to the inverter to reverse the polarity of the normal power pulse that would be provided by the inverter to force commutate off the SCR in the static switch which is conducting, and   wherein the inverter H-bridge has four arms with switching devices in upper and lower arms of the bridge, and with the DC input to the bridge from the battery being provided across the upper and lower arms, and with the output of the bridge being taken between the junctions between the switching devices in the upper and lower arms on each side of the bridge, and wherein at least the lower arms of the bridge include parallel connected plural gate controlled switching devices with the number of switching devices in each arm of the upper arms of the bridge being less than the number of switching devices in each of the lower arms of the bridge, and wherein the control means controls the switching of the switching devices such that between pulses of output power provided from the inverter to the power supply path, the control means turns off the switching devices in the upper arms of the bridge and turns on the switching devices in the lower arms of the bridge to allow current flowing from the inverter to the load to freewheel through the switching devices in the lower arms of the bridge, whereby the overall duty cycle for individual switching devices in the upper and lower arms of the bridge over a full cycle of the inverter tends to be equalized.   
     
     
       29. The uninterruptible power system of claim 28 wherein the switching devices in the H-bridge inverter comprise power FETs. 
     
     
       30. An uninterruptible power system connectable to AC power system lines at its input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) switch means connected in the power supply path and responding to control signals for interrupting the supply of power through the power supply path from the AC power system lines to the load;   (d) a main transformer having a primary winding and a secondary winding, the secondary winding connected at its terminals to the power supply path between the switch means and the output terminals so that power can be provided from the secondary of the transformer to the load when the switch means has interrupted the supply of power from the AC power system lines to the load;   (e) an inverter connected to receive DC power from the battery and connected to selectively provide AC power to the primary of the transformer when backup power is to be supplied to the load and the switch means interrupts the connection between the input terminals and the output terminals;   (f) rectifying means for rectifying the voltage appearing at the primary of the transformer as a result of the AC voltage across the secondary of the transformer when AC power is supplied through the power supply path from the input terminals to the output terminals and the inverter is not providing AC power to the primary, the rectifying means providing unidirectional voltage at a level nominally below the voltage level of the battery when fully charged;   (g) boost converter means, connected to receive the rectified voltage from the rectifying means, for providing charging current to the battery, the boost converter means including an inductor connected in a conduction path between the rectifying means and the battery and a controllable switch connected between the inductor and a battery common return line and including means for sensing the voltage across the battery and the current flowing through the inductor, the boost converter means determining when the voltage across the battery drops below a selected level and then closing the switch to draw current through the inductor and switch until the current in the inductor reaches a selected level, then opening the switch to cause the current in the inductor to flow through a forward biased diode to the battery to charge the battery for a selected period of time, and thereafter again closing the switch to draw a current through the inductor and repeating the closing and opening of the switch in a cyclic fashion until the voltage sensed across the battery reaches a selected level and then maintaining the switch open.   
     
     
       31. The uninterruptible power system of claim 30 wherein the rectifying means includes rectifying diodes connected to each terminal of the primary of the transformer to unidirectionally pass current from one terminal or the other of the transformer to the boost converter means, wherein the inverter comprises gate controlled switching devices having parallel intrinsic diodes connected in an H-bridge configuration, with the DC power from the battery connected on a DC bus across one side of the battery and returned to the battery through a common connection, and the other side of the bridge connected to the two terminals of the primary of the transformer, and wherein the rectifying means includes the intrinsic diodes of the H-bridge switching devices as connected between common and the terminals of the transformer. 
     
     
       32. The uninterruptible power system of claim 30 wherein the intrinsic diodes of the switching devices of the H-bridge inverter connected between the DC bus line from the battery and the terminals of the primary of the transformer serve to limit the voltage applied by the rectifying means to the boost converter means to the voltage level of the battery. 
     
     
       33. The uninterruptible power system of claim 30 wherein the means for sensing the voltage across the battery and the boost converter means comprises a feedback path feeding back the output voltage across the battery through a resistance to the boost converter means for comparison with a desired voltage level and wherein the feedback path also includes a temperature sensitive resistor which changes resistance with temperature to provide compensation of the boost converter charging cycle for temperature. 
     
     
       34. The uninterruptible power system of claim 30 further including a voltage sense transformer having a primary connected to the power supply path between the input terminals and the switching means and a secondary which provides AC power at a reduced voltage from that appearing at the input terminals of the system, a rectifier connected to the terminals of the secondary of the voltage sense transformer which provides a unidirectional DC output voltage, the output voltage of the rectifier being connected to the inductor of the boost converter means in parallel with the voltage provided from the rectifying means connected to the main transformer, and a switch connected between the rectifier connected to the voltage sense transformer and the inductor of the boost converter means which is open so that no current flows therethrough when the uninterruptible power system is providing power to the load either through the main power supply path or from the inverter, and is closed to supply current therethrough when AC power is available from the AC power system at the input terminals and power is not being supplied to the load through the uninterruptible power system. 
     
     
       35. An uninterruptible power system connectable to AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) static switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load, the static switch means including parallel connected SCRs connected with opposite polarity in the power supply path and gate drivers connected to the gates of the SCRs;   (d) a relay having a mechanical relay contact connected in the power supply path between the input terminals and the static switch means, wherein the relay contact is opened or closed in response to a control signal provided to the relay;   (e) means connected to receive DC power from the auxiliary battery and for selectively providing AC power to the power supply path between the static switch means and the load;   (f) control means, connected to provide control signals to the SCRs of the static switch means, to the relay, and to the means connected to receive DC power from the auxiliary battery and provide AC power to the power supply path, and including means for sensing the voltage at the input terminals of the power supply system to determine when an AC power line fault occurs, and upon occurrence of a fault for providing control signals to inhibit the triggering of the SCRs and controlling the means connected to receive DC power to reverse bias an SCR of the static switch that may be conducting at that time and thereafter providing AC power from the battery to the load, and after the SCRs of the static switch means are non-conducting, providing a control signal to the relay to open the relay contact whereby the relay isolates the power supply path from the input, terminals of the uninterruptible power system.   
     
     
       36. The uninterruptible power system of claim 35 wherein the control means senses when there is no longer a line fault on the AC power system lines and provides a control signal to the relay to close the mechanical contact of the relay, and then discontinues the supply of AC power from the battery to the load and thereafter provides control signals to the SCRs of the static switch to turn them on to resupply power from the AC power system lines to the load. 
     
     
       37. The uninterruptible power system of claim 35 wherein the means connected to receive DC power from the battery and for providing AC power to the power supply path includes a transformer having its secondary connected in the power supply path between the static switch means and the output terminals and also having a primary, and an inverter bridge formed of gate controlled switching devices connected together in a bridge configuration connected to receive DC power from the battery and to provide AC power to the primary of the transformer. 
     
     
       38. An uninterruptible power system connectable to the AC power system lines at input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from an AC power system lines connected to the input terminals to a load connected to the output terminals;   (c) switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load;   (d) means connected to receive DC power from the auxiliary battery and for selectively providing AC power to the power supply path between the switch means and the load, including an inverter comprising gate controlled switching devices connected in an H-bridge configuration to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path, wherein the H-bridge has four arms with switching devices in upper and lower arms of the bridge, with the DC input to the bridge from the battery being provided across the upper and lower arms, and with the output of the bridge being taken between the junctions between the switching devices in the upper and lower arms on each side of the bridge, and wherein at least the lower arms of the bridge include parallel connected plural gate controlled switching devices with the number of switching devices in each arm of the upper arms of the bridge being less than the number of switching devices in each of the lower arms of the bridge;   (e) control means, connected to provide control signals to the switch means and to the gates of the gate controlled switches of the inverter means, and including means for sensing the voltage at the input terminals of the power supply system to determine when a power line fault occurs, and upon occurrence of a fault for providing control signals to the switch means to interrupt the connection of power from the AC power system lines to the load and providing control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion to generate an AC output waveform at the output terminals of the power supply system, and wherein the control means controls the switching of the switching devices such that between pulses of output power provided from the inverter to the power supply path, the control means turns off the switching devices in the upper arms of the bridge and turns on the switching devices in the lower arms of the bridge to allow current flowing from the inverter to the load to freewheel through the switching devices in the lower arms of the bridge, whereby the overall duty cycle for individual switching devices in the upper and lower arms of the bridge over a full cycle of the invertor tends to be equalized.   
     
     
       39. The uninterruptible power system of claim 38 wherein the switching devices in the H-bridge inverter compriseise power FETs. 
     
     
       40. The uninterruptible power system of claim 38 wherein the means connected to receive power from the auxiliary battery includes a transformer having its secondary connected in the power supply path between the switch means and the output terminals and its primary connected to the inverter bridge to receive AC power from the invertor bridge. 
     
     
       41. The uninterruptible power system of claim 38 wherein the control means controls the switching devices of the inverter in a pulse-width-modulated manner to provide a substantially sinusoidal output voltage wave at the output terminals with the width of the pulses provided from the invertor selected to provide the desired sinusoidal output waveform with the waveform selected to match the waveform of the AC power system line voltage at the time a line power fault is detected so that a substantially continuous output voltage waveform is provided at the output terminals during switch over from AC line power to power from the inverter. 
     
     
       42. The uninterruptible power system of claim 41 wherein the control means includes means for sampling the AC power line voltage at the input terminals at periodic times and providing digital data corresponding to the sampled input voltage, means receiving the digital data corresponding to the sampled power line voltage and for producing a reference waveform at selected sample times during the period of the AC voltage signal wherein each reference value at each sample time comprises a selectively weighted average of sample values of the cycle of the AC waveform presently being sampled and prior cycles of the AC power line voltage, means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power line voltage, and a digital-to-analog conversion means receiving the digital reference waveform data for providing an analog reference waveform signal corresponding thereto, and wherein upon detection of a power line fault the switching devices of the inverter are controlled in a pulse-width-modulated manner to provide an AC output waveform from the inverter which tracks a desired waveform comprising the reference waveform signal and thereafter the means receiving the digital data calculates the reference waveform values at each sample time as a selectively weighted average of a pure sinusoidal waveform stored in a memory and recalculates the reference waveform in this manner for each cycle of the output waveform from the inverter. 
     
     
       43. The uninterruptible power system of claim 42 further including means for sensing the output current supplied from the output terminals to the load, and wherein the desired output waveform provided by the control means is the reference waveform compensated by the measured current being drawn by the load. 
     
     
       44. The uninterruptible power system of claim 43 wherein the desired waveform VREF* is determined in accordance with the following expressions:   VREF*=VREF+R(IO+)-R(IO-)     when the reference voltage is in the positive half-cycle, and     VREF*=VREF-R(IO+)+R(IO-)     when the reference voltage is in the negative half-cycle, and where IO+ is the sensed output current in a positive direction, IO- is the sensed output current in a negative direction, and R is a resistive scaling factor.   
     
     
       45. An uninterruptible power system connectable to AC power system lines at its input terminals and to a load at its output terminals comprising: (a) an auxiliary power supply battery;   (b) a power supply path from the input terminals to the output terminals to normally supply power from AC power system lines conncted to the input terminals to a load connected to the output terminals;   (c) switch means connected in the power supply path for responding to control signals to interrupt the supply of power through the power supply path from the AC power system lines to the load;   (d) means connected to receive DC power from the auxiliary battery and controllable for selectively providing AC power to the power supply path between the switch means and the load;   (e) control means connected to provide control signals to the switch means and to the means for selectively providing AC power to the power supply path, and including means for sensing the voltage at the input terminals to determine when an AC power line fault occurs, and upon occurrence of a fault, for providing control signals to the switch means to interrupt the supply of power from the AC power system lines to the load and to provide control signals to the means for selectively providing AC power to the power supply path to provide an output voltage waveform at the output terminals selected to match substantially the waveform of the AC input power at the time a line power fault is detected so that a substantially continuous output voltage waveform is provided at the output terminals during switch over from AC line power to power obtained from the battery, wherein the control means includes means for sampling the AC power line signal at the input terminals at periodic times and providing digital output data corresponding to the sampled power line voltage, means receiving the digital data corresponding to the sampled power line voltage signal and for producing a reference waveform at selected sample times during the period of the AC power line voltage wherein each reference value at each sample time comprises a selectively weighted average of sample values of the cycle of the AC waveform presently being sampled and prior cycles of the AC power line voltage, means for providing digital data corresponding to the reference waveform data synchronized in frequency and phase to the AC power line voltage, and digital-to-analog conversion means for receiving the digital reference waveform data and providing an analog reference waveform signal corresponding thereto, and wherein upon detection of a power line fault, the means for selectively providing AC power to the power supply path provides an AC output waveform which tracks the reference waveform signal.   
     
     
       46. The uninterruptible power system of claim 45 wherein after detection of a line fault, the means receiving the digital data calculates reference waveform values at each sample time as a selectively weighted average of a pure sinusoidal waveform stored in a memory and the prior reference waveform and recalculates the reference waveform in this manner for each cycle of the output waveform. 
     
     
       47. The uninterruptible power system of claim 43 including means for comparing the voltage across the switching devices in upper arms of the H-bridge inverter to a reference voltage indicative of a maximum current flowing through the switching devices and therefore a maximum voltage drop across the switching devices, and providing an output signal if the voltage across the switching devices exceeds the reference, the control means terminating the pulse of the pulse width modulation by turning off the switching devices when the current limit signal occurs. 
     
     
       48. The uninterruptible power system of claim 45 wherein the means for selectively providing AC power to the power supply path includes an inverter comprising gate controlled switching devices connected in an H-bridge configuration with upper and lower arms to receive DC power across one side of the bridge from the auxiliary battery and provide AC power across the other side of the bridge to the power supply path, and wherein the control means provides control signals to the gates of the switching devices of the inverter bridge in a pulse-width-modulated fashion with the width of the pulses provided from the inverter selected to provide the desired output waveform.